Commemorations started with a Catholic church service at the Field Cathedral of the Polish Army in Warsaw.

Participants in the day's events, among them the prime minister and Speakers of both houses of the Polish parliament, placed wreaths and lit candles at the Tomb of the Unknown Soldier in the Polish capital.

It was the second time Poland marked the national day after it was instituted in 2016.

It commemorates the victims of genocidal killings, known as the Volhynia Massacres, which were carried out between March 1943 and the end of 1944 by the Ukrainian Insurgent Army (UPA) in Nazi German-occupied Poland, according to Poland’s National Institute of Remembrance (IPN).

The massacres were part of the Ukrainian Insurgent Army’s plan to have a sovereign and nationally homogenous Ukraine after the war.

The IPN, which is charged with prosecuting crimes against the Polish nation, has said some 100,000 Poles died in the massacres, mainly women and children as men had already been subjected to mass deportations and repressions both by Soviet and Nazi authorities by the time the massacres started.

Meanwhile, some 10-12,000 Ukrainians were killed in revenge attacks by Poles, the IPN has said. Poland's IAR news agency added that some Ukrainians were killed by Poles acting in self-defence, and by other Ukrainians, in retribution for their attempts to help Poles.

The national day of remembrance coincides with the anniversary of Bloody Sunday, 11 July 1943, possibly the bloodiest day of the Volhynia Massacres, when the UPA attacked 100 villages largely inhabited by Poles in what was then Nazi-occupied eastern Poland and is now western Ukraine. (vb/pk)